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Constantine III, "the
usurper"

He was proclaimed emperor in Britain by the
legions in AD 406. His (honorific) name referred to Constantine
the Great and expressed his ambitions. In AD 411 his adventure
eventually failed and he was beheaded by Constantius, emperor Honorius'
general. It is thought that Constantine had left so few troops in
Britain that the isle became easy prey for Saxon and other raiders
(Saxons = Vikings).

Shortly after Constantine III went to the continent, the 'Roman' (tax) administration
was
destituted. It is probable that he had appointed many
civil servants (of reliable Welsh origin) to enforce his authority. One
of the main tasks of those civil servants was to collect taxes. Tax
collecting had to be fair and independent. In reality, the very same
south-Welsh families who used to export goods to the continent, and
fixed the purchase prices in Britain also collected taxes. This led
inevitably to corruption.

At that moment, all taxes were sent to the
continent in support of Constantine’s ambitions. Britain was deprived
of the means to defend itself against the raids. As a result, a part of
the upper class revolted and the tax administration was made destitute.
The revolt happened mainly in the most threatened eastern part of
Britain. People there must have blamed the Welsh for their futile
ambition and corrupt attitude. This added to the age-old reciprocal
resentments.

It's not so that taxes were no longer collected.
They were simply kept in Britain, locally in the hands of the British
lords.

In both wars (Maximus and Constantine), it is
very likely that most finances were provided by southwestern Welsh
lords who hoped for a renewal of the Empire. Unthreatened by raids, the
southwest lords remained loyal to the imperial ideal. The Empire always
had been a major source of income for them. The eastern lords had
learnt a completely different lesson.

Constantine III had left so few soldiers in
Britain that there was a shortage of skilled soldiers in Britain. To
overcome this problem, many east-British lords began to hire warriors
from northern Germany. To finance the building of those local guards,
tax money was used.

This means that there were already Anglo-Saxons
coming to Britain from at least 407-408.